The joining operation of underwater pipelines to be laid on the bed of a body of water comprises welding the metal cylinders together, normally with a plurality of weld passes; and coating the cutback. Once an annular weld bead is formed between two adjacent metal cylinders, the cutback extends astride the annular weld bead, along an uncoated portion. In other words, the cutback is substantially defined by the free ends of the pipes, extends axially between two end portions of the protective coatings, and must be coated with a protective coating to prevent corrosion.
Coating the unprotected portion along the cutback is known as ‘Field Joint Coating’, and comprises coating the cutback with normally three coats of polymer material, to protect and ensure adhesion of the coats to the metal cylinders.
For example, coating the unprotected cutback comprises heating the cutback, such as to a temperature of 180° C.-250° C.; spraying the cutback with powdered epoxy resin (FBE—Fusion Bonded Epoxy) which, in contact with the cutback, forms a relatively thin first coat or ‘primer’ measurable in tenths of a millimeter (mm), spraying over the first coat on the cutback a modified adhesive copolymer which, in contact with the first coat, forms a relatively thin second coat, also measurable in tenths of a mm; and applying a third so-called ‘top coat’ which overlaps the pre-existing protecting coatings.
Welding, non-destructive weld testing, and coating the unprotected cutback are carried out at work stations equally spaced along the path of the pipes (or of the pipeline being constructed, if the pipes are joined to the latter). So the pipes are conveyed in steps, and stopped for a given length of time at each work station.
One known method of applying the third coat is to wind thick, 2-10 mm thick, protective sheeting about the cutback. The protecting sheeting is extruded, and is simultaneously wound about the cutback as it is formed. This method is described in the Applicant's PCT Patent Application WO 2008/071773 and in EP Patent Application EP 1,985,909, PCT Patent Application WO 2010/049353, and PCT Patent Application WO 2011/033176. In the methods described in these documents, the protective sheeting is applied by an extrusion outlet mounted on a carriage which runs along an annular path about the longitudinal axis of the pipeline, and the polymer material is plastified by a plastifying device located close to the pipeline and either connectable selectively to the extrusion outlet, or connected to it by a hose or semirigid pipe.
The protective sheeting is applied over the second coat and over two opposite end portions of the pre-existing coatings, and is pressed onto the pipeline to make sure it adheres properly. Because the end portions of the pre-existing coatings define raised areas with respect to the cutback, the surface onto which the protective sheeting is applied is not perfectly flat, so it is difficult to ensure the roller exerts even pressure on the sheeting as a whole.
In the case described, the weld bead also defines a ring with respect to the adjacent parts of the cutback. As such, the types of rollers described in certain of the known art fail to ensure even pressure over the protective sheeting as a whole.